From Wikipedia, the free encyclopedia - View original article
CAS Registry Numbers are unique numerical identifiers assigned by the Chemical Abstracts Service to every chemical described in the open scientific literature (currently including those described from at least 1957 through the present) and including elements, isotopes, organic and inorganic compounds, ions, organometallics, metals, nonstructurable materials (aka 'UVCB's- i.e., materials of Unknown, Variable Composition, or Biological origin). They are also referred to as CAS RNs and CAS Numbers.
The Registry maintained by CAS is an authoritative collection of disclosed chemical substance information. Currently the CAS Registry identifies more than 71 million organic and inorganic substances and 64 million protein and DNA sequences, plus additional information about each substance. The Registry is updated with an approximate 15,000 additional new substances daily. 
Historically, chemicals have been identified by a wide variety of synonyms. Frequently these are arcane and constructed according to regional naming conventions relating to chemical formulae, structures or origins. Well-known chemicals may additionally be known via multiple generic, historical, commercial, and/or black-market names.
On the other hand, CAS Numbers are not related to chemistry, are unrelated to any previous systems, and do not readily form phonetic analogs or synonyms. The numbers are simple and regular, convenient for database searches.
They offer a reliable, common and international link to every specific substance across the various nomenclatures and disciplines used by branches of science, industry, and regulatory bodies. Almost all molecule databases today allow searching by CAS Registry Number.
A CAS Number has no inherent meaning but is assigned in sequential, increasing order when the substance is identified by CAS scientists for inclusion in the CAS REGISTRY database.
A CAS Registry Number is separated by hyphens into three parts, the first consisting of up to 7 digits, the second consisting of two digits, and the third consisting of a single digit serving as a check digit. The check digit is found by taking the last digit times 1, the previous digit times 2, the previous digit times 3 etc., adding all these up and computing the sum modulo 10. For example, the CAS number of water is 7732-18-5: the checksum 5 is calculated as (8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6) = 105; 105 mod 10 = 5.
The assigning agency, Chemical Abstracts Service (CAS) is a function of the American Chemical Society (ACS); CAS information is copyrighted by the ACS. Users wishing to incorporate CAS Numbers into databases should refer to CAS policy:
A User or Organization may include, without a license and without paying a fee, up to 10,000 CAS Registry Numbers or CASRNs in a catalog, web site, or other product for which there is no charge. The following attribution should be referenced or appear with the use of each CASRN: CAS Registry Number is a Registered Trademark of the American Chemical Society.
To find the CAS number of a compound given its name, formula or structure, the following free resources can be used: